This invention relates to automotive vehicle mufflers and methods of reducing engine exhaust noise.
Exhaust mufflers have been used in automotive applications to reduce engine exhaust noise and smooth exhaust-gas pulsations. Traditional mufflers may consist of a series of fixed expansion or resonance chambers of varying lengths connected together by pipes. The exhaust noise reduction is determined by the size and shape of the expansion chambers. Additional expansion chambers may be added to reduce exhaust noise; however, the additional chambers increase exhaust backpressure. Mufflers may also include sound deadening material which dampens sound over a broad band of higher frequencies. Consequently, lower sound frequencies are not reduced.
The present invention provides a muffler having a variable length resonance chamber. The variable chamber is defined by a slidable member that is slidable to vary the volume and length of the chamber. Changing these characteristics of the chamber tunes the muffler to provide variable acoustic resonance, while maintaining exhaust efficiency. The interior of the muffler may be divided into two or more resonance chambers. A spring or other biasing member preferably urges the slidable member toward an initial position from which it is moved by increasing exhaust flow pressure. Alternatively, other means for moving or positioning the slidable member may be utilized.
At idle, engine noise is nominal; therefore the muffler is only required to provide minimal sound dampening. At this speed, the slidable member is positioned to provide minimal noise reduction and maximum flow efficiency. As engine speed increases, additional exhaust gas flows into the muffler. The increasing gas flow pressure actuates the slidable member, against the spring force, thereby increasing the length of the resonance chamber and changing the flow path of the exhaust gas. The increased length in the resonance chamber improves acoustic cancellation, which reduces the intensity of the exhaust noise. As engine speed and exhaust noise are reduced, the biasing device moves the slidable member back toward its original position. This decreases the length of the resonance chamber, which reduces acoustic cancellation and decreases backpressure in the muffler.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.